It was a warm day in Ohio yesterday and I took my 1975 with 125,000 miles 914 out for a ride. As I stepped on the brake the pedal went to the floor and after a few pumps the brakes took hold. I slowly drove home wondering what was up. Eight years ago replaced all the brake lines but not the master cylinder. Last night I quickly inspected all the calipers and the fluid reservoir. All seemed OK, The brakes now feel spongy. Am I correct in my assessment that I need to replace the master cylinder, or is there something else I should take into consideration. I doubt that it is air in the line for that would contribute to sponginess, but not cause the pedal to go all the way to the floor without stopping the car.

Thanks in advance for any advice

Certainly sounds like the master cylinder may have failed,have you checked around the pedal for leaks ?

Exactly what my master cylinder did before it completely failed

Yup. Master cylinder. The stock Ate 17mm can be bought at Pelican Parts. Make sure you bench bleed before you install. I did not, and it required a pressure bleeder to get the air out.